1. Field of the invention
The present invention relates generally to composite molding processes for forming composite bodies that include a fabric. More specifically, the present invention relates to a process and product for improving composite molding processes and the composite bodies resulting therefrom.
2. Background Information
A number of composite molding processes for forming composite bodies including a fabric are known. These processes, and the products resulting from them, find utility in the aerospace industry and other industries where parts of complex shape having the combination of high strength and low weight are advantageous.
One such composite molding process is resin transfer molding ("RTM"). In RTM precesses, a fabric material, such as fiberglass or woven carbon fibers, is placed on or laid-up in a die or mold tool having the contours of the desired part therein. After the fabric is laid-up in the molding tool, the tool is placed in a molding press in which a resin material is injected in a fluid state to impregnate and fill the fabric and cavities in the die or mold tool. After the resin is injected and cured, a composite body or part results that has excellent strength and generally low weight. RTM and similar composite molding processes also are very cost-effective.
If the die or molding tool possesses many contours, and particularly upstanding walls, the lay-up of the fabric in the mold, prior to the resin injection step, is rendered difficult because the dry fabric does not adhere the contours of the die or mold tool, particularly to upstanding surfaces. If the fabric is not laid-up properly, the resulting composite body will not possess adequate strength due to wrinkles or other imperfections in the laid-up fabric.
Prior-art solutions to this problem include tackifying, or rendering tacky, the fabric prior to the lay-up step of the composite molding process. Generally, use of an adhesive in the tackifying process that is chemically different from the resin ultimately injected in the composite molding process is unsatisfactory. This is because an adhesive having chemical and other properties different from that of the injection resin can degrade the strength of the cured resin and the resulting composite body.
As a general matter, most injection resins are too viscous in their uncured state to be applied to a fabric to provide a satisfactory tackifying material. Therefore, prior-art tackifying processes have employed the injection resin, thinned with an organic solvent such as acetone, as the tackifying adhesive. The resin, thinned with the organic solvent, is of a satisfactory consistency to provide tackification. However, as with adhesives chemically different from the injection resin, the organic solvent present in the tackifying material degrades the cured resin and the strength of the resulting composite body.
A prior-art alternative to the use of solvent with the injection resin as a tackifier is to provide the injection resin treated in such a manner that it is a fine powder. This powder can be applied to the fabric and serves as a tackifying adhesive. Once such product is and sold under the tradename AMD, sold by 3M Corporation of St. Paul, Minn. This product is a derivative of, and therefore chemically and otherwise compatible with, the injection resin sold under the tradename PR500 epoxy resin, also manufactured by 3M Corporation of St. Paul, Minn. However, because the AMD product is in a fine, powdered form, it becomes airborne easily, and could pose a respiratory health hazard. Currently, tackifying processes employing the AMD material are conducted in a controlled environment with all operators wearing cumbersome respirator equipment.
In addition to the inadequacy of prior-art tackifying materials, the processes employed in applying the tackifying material to the fabric have been less than satisfactory. In prior-art processes, the tackifying material was applied by manual painting or spraying. These prior-art processes allow little or no control over the amount of tackifying material being applied to the fabric. Moreover, spraying the tackifying material causes it to become airborne, increasing the risk of a respiratory health hazard.
A need exists, therefore, for a process for tackifying fabric for use in a composite molding process that employs the unadulterated injection resin as the tackifying material and that controls the amount of tackifying composition applied to the fabric. A need also exists for a tackified fabric for use in composite molding processes.